This invention relates to a thin-film transistor (from here on will also be referred to as a TFT) which is made of non-single-crystal semiconductor, for example an IG-FET, and its manufacturing process, and in more particular, to a highly reliable thin-film transistor which is suitable for use as a driving element of a display image sensor or liquid crystal device or the like.
Thin-film transistors can be formed by a chemical vapor deposition method on an insulated substrate in a comparatively low temperature atmosphere, with a maximum temperature of 500° C., and the substrate being made of an inexpensive material such as soda glass or boron-silicate glass.
This thin-film transistor is a field-effect transistor and has the same features as a MOSFET. In addition, as mentioned above, it has the advantage that it can be formed on an inexpensive insulated substrate at a low temperature. Also the thin film transistors can be formed on a large substrate by the use of CVD techniques. It is therefore a very good prospect for use as switching elements of a matrix type liquid crystal display having a lot of picture elements, or as switching elements of a one-dimensional or two-dimensional image sensor.
Also, the thin-film transistors can be formed using already established photolithography technology, by which a very minute process is possible, and transistors can be integrated just as making an IC and so on. FIG. 1 shows the construction of a typical prior art TFT.
In FIG. 1, the thin-film transistor is comprised of an insulated substrate 20 made of glass, a semiconductor thin film 21 made of a non-single-crystal semiconductor, a source 22, a drain 23, a source electrode 24, a drain electrode 25, gate insulating film 26, and a gate electrode 27.
In this type of thin-film transistor, the current flow between the source 22 and the drain 23 is controlled by applying a voltage to the gate electrode 27. The response speed of the thin-film transistor is given by the equation; S=μ·V/L2 where L is a channel length, μ is a carrier mobility, and V is the gate voltage.
In this type of thin-film transistor, the non-single-crystal semiconductor layer contains many grain boundaries. The non-single-crystal semiconductor, when compared to the single-crystal semiconductor, has disadvantages that the carrier mobility is very low and thus the response speed of the transistor is very slow due to the many grain boundaries. Especially if an amorphous silicon semiconductor is used, the mobility is only about 0.1-1(cm2/V.sec) and is too short to function for use as a TFT.
It is obvious that to solve this problem the channel length needs to be shortened and the carrier mobility increased. Many improvements are being made.
When the channel length L is decreased, the effect it has on the response speed is as the square of the length, and so it is a very effective means. However, when forming elements on a large area substrate, it is apparently difficult to use the photolithography technique in order that the space between the source and drain (this is essentially the channel length) should 10μm or less, due to the precise process, yield, and manufacturing cost problems. Consequently, effective means for shortening the channel length of the TFT have not been found.
On the other hand, to increase the mobility (μ) of the semiconductor layer, single-crystal semiconductor or poly-crystal semiconductor material is used, and when using amorphous semiconductor material, after the semiconductor is formed, the active region of the TFT should be crystallized using a process such as heat treatment.
In this case, a temperature higher than what is normally required to form a-Si is necessary. For example;    (1) For a thin-film transistor made of amorphous semiconductor material, the amorphous silicon film is made at a temperature of about 250° C. and then a maximum temperature of 400° C. is required for thermal annealing.    (2) When a poly-crystal silicon film is formed by a low pressure CVD method, the maximum temperature required for forming the film and then for recrystallization is 500 to 650° C.    (3) For a thin-film transistor where only an active layer is converted to a poly-crystalline structure, the required CVD temperature for forming the semiconductor layer is 250 to 450° C., however the temperature exceeds 600° C. during a recrystallization step of the active layer by CW laser.
The TFT is formed on a substrate made of a material such as soda glass and the active region comes in direct contact with the glass substrate, especially in the case of stagger-type and coplanar-type transistors. When making a TFT that has sufficiently fast response speed, the heat treatment mentioned above is necessary, and so the metallic alkali impurities such as sodium and potassium which exist in the glass substrate are externally diffused and forced into the semiconductor layer which forms the active layer or TFT. This lowers the mobility of the semiconductor layer and changes the threshold value, making the characteristics of the device worse and has an adverse effect on the long-term reliability of the device.
Also, through operation of the TFT, the TFT produces heat which causes the temperature of the glass substrate to rise thus causing impurities to be diffused from the substrate, which also has an adverse effect on the TFT.
Generally, a gate-insulator of the IG-FET is made of a silicon oxide film which is formed by a sputtering method with argon (Ar) gas used as a sputtering gas. In the sputtering process, the argon atoms are inherently introduced into the gate insulator and generates a fixed charge in the semiconductor film. Also, ions that exist in a reaction space during the sputtering collide with the surface of the active layer of the thin-film transistor, which causes a damage to the active layer. As a result, a mixed layer of the active layer and the insulation layer is formed in the boundary region of the gate insulation layer and the active layer of the transistor. In producing a TFT as described above, the problems of response speed and reliability need to be solved.